Accurate determination of the bed level of settled particles is important to control settling tanks such as clarifiers, thickeners, and aluminum and coal settlers. During operation of a settler, a slurry formed of a liquid laden with particles such as aluminum, solid wastes or coal is carried via a pipe to the center top, i.e., the "center well" of a settling tank. The slurry then is discharged into the center well, and the particles are allowed to settle onto the bottom of the tank. The settled particles form a bed, and the liquid-solid interface between the waste suspension and the liquid above is the bed level. Often, however, the particles in the slurry do not immediately settle to the bottom, but remain suspended in the tank to form a "cloud" that can create or be a predictor of an upset condition. Knowledge of the location of bed level and/or cloud dimensions and intensity is important for controlling the amount of chemicals or flocculants added to the tank to control the settling process.
Settling rates in thickeners and clarifiers have been manually controlled due to lack of reliable measurement instrumentation. Settling rates have been determined manually by depositing a sample of the slurry taken from the settling tank into a graduated cylinder, and employing photo cells or visual observation to measure the time for the suspended particles to fall a given distance within the cylinder. This method, although simple and inexpensive, is not a reliable means of measuring settling rates since it depends on obtaining a representative sample from the settler, is performed outside the settler, and its accuracy depends upon a human observer.
The bed level of settled particles also has been determined by using simple non-coherent fish finder (A-Mode) ultrasound systems. In an A-mode system, a transducer sends an ultrasound pulse into the particle suspension contained in a "settler". Low level echoes return from the surface of the settled bed. If the speed of the ultrasound pulse is known, then the distance to the bed level of settled particles can be calculated from the time between the transmitted pulse and the returned echo by using the well known range equation: EQU d=ct/2
where d=distance to the target, PA1 c=speed of sound in the liquid or other media PA1 t=round trip time from the transducer pulse to echo return.
The simple A-mode systems of the art are useful when a distinct bed level boundary exists and where that boundary is essentially perpendicular to the ultrasound transducer path. However, if the bed level boundary is not nearly perpendicular to the transducer path, or when there is no distinct bed level boundary, then echoes from the bed level may be blurred or undetectable. Moreover, in the unsettled particles where a bed level might exist, A-mode ultrasound systems provide little or no information on settler performance.
Currently, non-coherent A-mode systems cannot reliably detect bed level, cloud layer and cloud layer characteristics. A need therefore exists for reliable and accurate determination of bed level and cloud layer existence and particle activity within the cloud layer.